Wireless networks are often configured to support users having different QoS requirements. Such networks typically use a scheduler, implementing a designated scheduling algorithm, in order to meet the QoS requirements.
The term “scheduler” is used herein in a general sense, and accordingly is intended to encompass resource managers which implement various types of resource allocation algorithms. Such resource allocation algorithms may include, in addition to user selection or user ordering algorithms, power control algorithms, rate control algorithms, transmission rate and encoder format selection algorithms, sub-channel allocation algorithms, frequency hopping algorithms, and so on, in any combination.
If the number of users in the network is not too large, the scheduler is usually able to satisfy all the QoS requirements. However, as the number of users in the network increases, it becomes increasingly important to limit the number of users allowed to access the network and to select an appropriate set of users to ensure that the QoS requirements can be satisfactorily met while at the same time maximizing the utilization of the available network resources. Thus, admission control algorithms are used to ensure that the number of users in the network does not exceed the critical limit beyond which the QoS requirements can no longer be satisfied.
The admission control problem is fairly straightforward in wireless circuit-switched networks, but becomes more difficult in wireless packet data networks. The fundamental reason for the added difficulty is that the resources needed to meet the QoS requirements in wireless packet data networks depend not only on the number of users and their QoS requirements, but also on additional factors, such as the locations, channel qualities and traffic patterns of the users. The issue is further complicated by the fact that the mix of applications present in the network is typically varying over time, such that there is no set limit on the maximum number of users that can be admitted to the network.
Current solutions to the admission control problem have generally considered the problem from a circuit-switched point of view, in which a predetermined number of circuits or other channels are available and have to be shared between the competing users. The basic underlying assumption is that each user consumes a finite number of channels, which is fixed and independent of the user location and channel conditions, as well as the locations and channel conditions of the other users. In addition, the QoS requirements in the circuit-switched context typically include the probability of blocking a new call or the probability of dropping a handoff call. Once a user is accepted to the network, it is assumed that the achieved performance is acceptable to the user and the associated application. However, these assumptions are generally not valid in wireless packet data networks that rely on opportunistic scheduling algorithms to allocate the available resources between competing users.
Furthermore, current admission control techniques often consider only such limiting resources as the processing power of the base station, the number of codes available, the channel elements available, the memory and buffer requirements, or the backhaul capacity. However, in a wireless packet data network which supports multimedia applications, the performance as perceived by each application (e.g., in terms of factors such as the minimum achieved throughput, the average throughput, the average and the maximum delay, or the packet loss rate) is of more importance and is in fact expected to limit the number of concurrent applications that can be supported by the network.
It is therefore apparent that current admission control techniques are often based on assumptions which are not readily applicable to wireless packet data networks, and as a result such techniques do not adequately take certain constraints and requirements into account when deciding on the admission or rejection of a new user.
The above-cited U.S. patent application Ser. No. 11/156,752 addresses the drawbacks of conventional practice by providing, in an illustrative embodiment, an improved admission control approach involving the use of a virtual scheduler. The virtual scheduler emulates the operation of an actual scheduler in order to generate a performance metric for a system in which a new user is assumed to be admitted. The performance metric is used to make an admission control decision regarding admission of the at least one additional user to the system.
However, in some applications, the virtual scheduler approach may require more processing time, and associated memory and processor resources, than are available for implementing admission control functionality. Also, the virtual scheduler approach in the illustrative embodiment noted above generally requires explicit knowledge regarding the detailed operation of the underlying scheduling algorithm.
Thus, despite the considerable advances provided by the virtual scheduler approach, a need remains for alternative approaches which can alleviate one or more of the drawbacks of the conventional approaches as outlined above.